Ultrasonication enables achievement of reactions under normal conditions, which otherwise would require harsh reaction conditions (Ashokkumar et al., 2007). Ultrasound consists of pressure waves exceeding 18 kHz (Mason, 1997). Imparting this energy to a solution leads to acoustic cavitations (bubble formation) and subsequent implosion. The acoustic cavitation provides a unique interaction of energy and matter and is the foundation of sonochemisty and sonoluminesense (Suslick, 1998). Extremely high local temperature and pressure gradients are generated upon the collapse of the bubble. The estimated hotspot temperature and pressure has been found to be around 5000° C. and 1000 atm respectively (Mason, 1997; Suslick, 1998; Run et al., 2004). The heat generated is normally dissipated with no appreciable change in the environmental conditions and the cooling after the collapse of the cavitation bubbles is estimated to be around 10 billion ° C. per second (Dolores et al., 2007). Ultrasonic irradiation causes physicochemical effects in solid, liquid or even air (Enomoto et al., 1996) and has been reported to induce a stirring effect in a solution due to the microjetting and microstreaming of the collapsed bubbles (Lindley, 1992).
The sonochemical effects have been shown to have many different applications in research. For example; acceleration of the reaction in polymer synthesis due to formation of reactive intermediates (Michel, 2001), sample digestion (Priego-Capote and Luque de Castro, 2007a), synthesis of nano- and micro-particles (Lindley, 1992), intercalation and impregnation of nanoparticles to supports such as clays (Belova et al., 2009) and sonocrystallisation (Mason, 1997; Luque de Castro and Priego-Capote, 2007b). During the ultrasonic assisted sample digestion, the main advantages over other methods are the speed of digestion and high sample treatment capacity (Levilla et al., 2006). It also helps to prevent the loss of volatile elements such as, As, Se, and Hg (Aki and Ari, 2007).
Ultrasonic enhanced crystallization is not a new process and has been in use for many years (Lindley, 1992). Sonocrystallization accelerates nucleation and crystallization rates (Dolores et al., 2007, Luque de Castro and Priego-Capote, 2007b) and hence leads to improvement of the yield and particle size distribution. During ultrasonic assisted zeolite synthesis, ultrasound has been shown to increase the solubility of the feedstock into the mother liquor, influencing the depolymerisation and polymerisation kinetics, and also aiding in the crystallisation process (Jianmei et al., 2006). Although the mechanism of zeolite formation by the use of ultrasound is not well understood, Lindley (1992) proposed that during the expansion stage of the cavitation bubble there is localized cooling which in turn leads to localised increase in the degree of supersaturation that trigger the formation of a germ nuclei that is distributed through the solution upon the collapse of the bubble which later grows to form the crystalline material. This proposal is also backed by Luque de Castro and Priego-Capote (2007b).
Studies conducted by Feng et at (2004) showed that dissolution of fly ash and metakaolin was enhanced by the application of ultrasound. Studies of effects of ultrasound on synthesis of zeolite A conducted by Özlem et al. (2005) concluded that it was possible to obtain highly crystalline zeolite A from pure industrial chemicals on application of ultrasound.
Although ultrasonic enhanced crystallization as shown above is not a new process, there is no evidence in the literature reporting synthesis of zeolites by direct sonication of non-fused coal fly ash. Prior studies as reported by Belviso et al. (2011) describe indirect ultrasonication of fused fly ash prior to conventional hydrothermal treatment.
A need exists to address the shortcomings of the popular hydrothermal synthetic approaches by minimising high consumption of energy and prolonged synthesis time. In particular, a need exists to not use a separate hydrothermal process based on ultrasonic treatment